4hp single phase motor problem 1

[jemjack]

I have a high pressure compressor for filling air cylinders for diving, and a little while ago I asked about 5 hp motors for it, and got many helpful replies from you , thanks very much. I did have a mental block, as when I read the blurb a bit more carefully I saw that it really only required a 4 hp motor. Sorry about that.

The compressor at startup runs very freely and in the past I was using it with a 3 hp motor which worked well but after 40 minutes or so the motor got very hot, so I had to stop it. As time went on the motor would cut out after less and less time until it would only run for 15 minutes which was not enough time to fill a bottle, and I felt that soon I would burn out the motor! I had it connected through a DOL and it was this that cut out, presumably because the amperage rose to high?

I have just bought a new 4hp motor 220 volts ( I live in Spain) single phase 3000 rpm, which is the required motor for this compressor. Also new DOL for it.

The compressor is supposed to run at 1300 rpm but my pulleys are letting it run at rather less, So to my reckoning it should not need 4 hp to drive it!

All connected up and when I press the green button it starts up and runs for about 5 seconds or so and blows the 25 amp fuse at the mains. By the sound of the motor, it is up to the speed that it will run at, i.e. it has stopped accelerating, but has not tripped out the centrifugal switch . This is the problem . which I would very much like some help on please.
The motor runs at 2900 rpm when it has nothing attached to it, the voltage is 215 volts and the centrifugal trips out with no problem.
When the compressor is attached to it by the belt and pulleys the motor runs up to 1950 rpm but the centrifugal switch does not trip out at this speed. The amperage is way off the scale of my meter which only reads up to 50 amps the voltage drops to 180 volts. and the compressor is running at 725 rpm. The pulley ratio is 1: 2.76 I realize that this is so because the started windings are still in circuit.

So it would seem to me that
1 the motor is not producing 4 hp or
2 the centrifugal switch should trip out , then the voltage would rise, and the currant would drop to somewhere around 20 amps. and the motor would produce more power, and accelerate to somewhere near 2800 rpm
3 Or am I missing something here.

I have no way of having 3 phase, the cables from motor to meter are heavy duty about 6 mm so there should be no volt drop in them at 20 amps or so. The start windings have a 400 µf condenser and the windings have a resistance of about .7? and the running windings have a 60 µf condenser and the windings are about 1.9 ?.

Any help or advice would be very gratefully received,
Best wishes

Jemjack

 

[desertfox]

Hi jemjack

I'm no expert however its of no surprise to me that the motors getting hot if the start windings are still in circuit, can you alter the centrifugal switch to cut out at the lower speed?
One thing you haven't mentioned in your post is the torque required for your system, its possible that the motor cannot produce the torque required to operate the load at the speed you require.
Remember its the load demand which determines the power output of the motor and you haven't told us what the load demand is.
One other possibility is that you have a faulty centrifugal switch but I doubt that if it opens correctly under no load.

desertfox

 

[wayne440]

At the motor (while starting) you read 180V, but what is the voltage on the other end of your cable nearer the meter?

 

[rasevskii]

Is the compressor unloaded during start-up, if not that may be the problem. Are there unloader valves on it?

rasevskii

 

[waross]

Are you trying to start the compressor against back pressure?
Does the compressor have a faulty unloading system?
The compressor depends on flywheel inertia to complete the compression cycle. If any load comes on the compressor before the compressor is running fast enough to develop adequate flywheel inertia it will stall or at the least stop accelerating.
Check the 400 uf capacitor by replacing it with a new 400 uf capacitor. Then if it won't start try both 400 uf capacitors in parallel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

As was mentioned your compressor might have an unloader problem. Your compressor can NOT start under any load.

Disconnect the compressor's output air line so the air is free to vent to atmosphere and start it. If it starts correctly you have an unloader problem.

Your system may have other problems too, but one thing at a time.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jemjack]

Thank you very much for all your help, I will check the voltage at the meter when starting up, though I think that it may well be the same, as well over 50 amps is asking a lot.

The compressor defiantly does not have an unloader problem, as it worked with the 3 HP motor before, (which was an English motor rated at 240 volts so was running way below its rated voltage) and the air line was unconnected. There is no back pressure. The compressor turns over quite easily. I don’t have any details about the torque required for the compressor, only that it should be supplied with a 3 kw single or 3 phase motor or a 3.7 kw gasoline motor.

I would not think that there was any way to change the centrifugal switch to a slower speed is there?

I will certainly check the 400 µf condenser, I have all ready checked it for resistance, leakage. And try doubling the capacitance, this seems a good idea, although the manufacture specified the 400 µf. Do you think that this would affect the guarantee?

Once again thank you for all your help

Jemjack

 

[waross]

Is the motor wired for the correct voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gattie]

Have you tried powering the motor from a different power supply?
Take it to a suitable repair shop & get them to test it.
You might have a problem with your building wiring system.
Bad or loose connection??

 

[desertfox]

Hi jemjack

The torque rating of the motor is most important in motor selection, I don't suppose you have a motor torque curve for the motor you purchased?

desertfox

 

[wayne440]

I don't know what is typical for Spain, but if 50 or so amps of starting current is "asking a lot" of your incoming service, there is a good chance that you have just found the issue.

 

[petronila]

Hello jemjack

I am agree with desertfox, the problem coul be maybe you buy an standard torque motor and really you need a high torque motor

In Spain the single-phase motors are clasified in standard , medium and high torque,if you don´t know about the hi-torq motor and the supplier sold you whats he think or what he have in the moment.(Inmediate delivery) then this could be the failure.

Also I will check the belt tension and pulleys alignment this two issues could cause problems like overloadding.

Attached you will find some good information.

Good Luck(Suerte tío)

Carlos

 

[jemjack]

Hi everyone thanks once again,

yes the motor is wired for the correct voltage, thanks.

I haven’t got a torque curve for the motor, but the specification is,3 kw 4 hp 18.2 Amp 2820 rpm, fp cos? 0.95, rend 79% ,ppc (nm) 79, Pa 2, PMa 1.7 1a ipc 7 wt 23.2 Kg,

Is this any help?

Carlos, the motor is “SERIE MONOFÁSICA DE ALTO PAR DE ARRANQUE” so I suppose that it is high torque, and the belt is correct and in line, so that is not the problem. But thank you for that information.

I am wondering is it is correctly wired up in the connection box? Its z wirings are 1.9 ? and permanently in circuit through the 60 µf condenser and the 400 µf condenser which should switch out after startup, Its u windings are .7? permanently in circuit in parallel with the z windings.

I would be very grateful if someone could give me a wiring dia for this type of motor.

Thank you once again for all your help.

Jemjack

 

[7anoter4]

If the motor rpm is less than 2900*.75=2175 rpm the centrifugal switch does not work and the current is elevated for longer than permitted. Since the motor alone can reach the rated speed the motor could not be the problem.
At the start, if the compressor is still not loaded the only load is the inertia moment [I think it is not so elevated].
If the supply source is not changed and the voltage drop is indeed = (215-180)/220=16% 4hp motor can start well.
Usually, if the compressor is rotating at low speed the cause could be a slipping belt. But in this case the motor speed is low and the transmitting ratio seems to be the rated.
I think the cause could be the compressor it self. The load is rising up when still in starting process-problem of valve, oil pump, filter and other.

 

[waross]

The start winding should be in series with the 400 uf cap. Both the winding and the cap should be disconnected by the centrifugal switch in the designs that I am familiar with.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

I think this is similar to what I have had but in that case I experienced, the unloader valve was the culprit! The unit I got relies on a timer and the timer was inadvertently turned earlier causing the motor to be loaded even when it has not reached the required speed. Tweaked the timer longer and the motor-compressor unit worked fine.

BTW, granting your unloader valve is okay, I did some quick calculations based on your data:

Your actual pulley ratio = 2.76

Required rpm for compressor = 1300 rpm

@a full-load speed of 2820 rpm, your compressor shaft speed is:
Compressor speed = 2820/2.76 = 1021.74 rpm.

The derated output = 1021.74/1300 = 78.6% of compressor CFM rating

Derated power required = 4 X (1021.74/1300)^3 = 1.94 hp; say 2 hp (95% efficiency)

Assuming you are filling air bottles with rated pressures of 3000 psi (207 bars) and you are using a compressor with a max pressure of 300 bars (4350 psi), your final pressure head at 1021 rpm is:

final pressure head = 3000 X (1021.74/3000)^2 = 1853 psi (127.76 bars) - definitely not a filled cylinder; a far cry from the 3000 psi you wanted!

Also, assuming your bottles have initial pressures of 50 psi before filling and you wanted it filled-up in 5 minutes (3000 psi, 3 gallons(11.2-liter bottles), you probably needed 352 liters per minute compressor (without air banks). If your compressor is of lower air capacity, expect a longer filling-up time, IMO.

Either you decrease the diameter of the driven pulley or increase the motor side pulley to hit a pulley ratio of 2.17.

 

[iop95]

Torque motor don't overpass load required torque and speed curve is limited at 1950rpm with high current from grid because low back-emf. I think you need to increase pulley ratio to 3.3 - 3.5 to overpass load torque demand even if in such way speed will be lower.

 

[jemjack]

Hi waross, that’s what I thought, but this defiantly is both coils left in circuit. Thanks burnt2x for all your information, luckily I am only filling to 150 ats most of the time, and I can do that in about 15 mins which is ok for me. The motor ran today, see below at 2500 rpm, so the compressor ran at 1000 rpm, by changing the motor pulley to a slightly bigger one I could have the compressor running at 1300 which is its correct speed, but perhaps it is better to leave things as they are ?

I have now got the compressor working, by overriding the centrifugal switch, I disconnected one wire of the 400µf compressor and touched it on the terminal for a couple of seconds, and then disconnected it, and the motor worked perfectly. It drew 13 amps at 220 volts and ran at 2500 rpm I ran it for about an hour and a half and at the end of this period the temperature of the casing of the motor rose from ambient 27ºc to 54ºc when I switched it off and the temperature then rose to a maximum of 63ºc.

This is a bit of a crude way of running the compressor, but there we are. I realise that the wiring would reach a much higher temperature than the 54ºc that I measured on the case, but is this reasonable? I presume that since the motor only drew 13 amps, it was not working very hard, this 4 hp motor should draw 18 amps! Also why did it only reach 2500 rpm it should reach about 2850 rpm shouldn’t it ?

Thanks once again for all your help

Jemjack.

 

[burnt2x]

Typical slip for that size of motor should be around 3-5%. Since the motor ran at 2500 rpm (16%), something is wrong with the running windings.

One more question here. Was the drive belt on when you took rpm measurement of 2500? Motor should be running at near synchronous speed (3000) when free. If it did not, then we can be sure something is wrong with your motor's running windings.

Good that your compressor works for now. Thanks for the feedback.

 

[jemjack]

Hi Burnt2x and everyone else who has been so helpful,

The motor was under load from the compressor when the rpm reading of the motor was 2500 rpm. A few days ago I tried the motor without the belts, and it ran perfectly, starting correctly and running at 2900, at 215 volts.

I suspect the windings or some other fault, as the compressor started perfectly before using the same pulley ratio and a uk 240volt 3hp motor, the trouble with this motor was that it got very hot after quite a short while. And the compressor specified a 4 hp motor.

Is 54 ºc reasonable for the case temperature of this type of motor? Please.

Thanks to everyone, best wishes

Jemjack.